ST7265X datasheet(68/166 Pages) STMICROELECTRONICS

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ST7265x

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USB INTERFACE (Cont’d)

11.3.4 USB Data Buffer Manager

The USB Data Buffer Manager performs the data

transfer between the USB interface and the two

512 Bytes RAM areas used for Endpoint 2 in both

Upload and Download modes. It also controls the

status of Endpoint 2, by setting the endpoint as

NAK when the current buffer is not yet available for

either transmission (Upload) or reception (Down-

load).

It is based on a stand-alone hardware state-ma-

chine that runs in parallel to the ST7 processing

flow. However, at any time, the ST7 software can

initialize the USB Data Buffer Manager state-ma-

chine in order to synchronize operations by writing

a ‘1’ to the CLR bit in the BUFCSR register.

Dedicated buffer status flags are defined to syn-

chronize the USB Data Buffer Manager with the

Data Transfer Coprocessor (DTC). These flags

are used by the software plug-ins provided by

STMicroelectronics) running on the DTC.

11.3.4.1 Data Transfer Modes

In USB normal mode (MOD[1:0]=00b), the maxi-

mum memory size of Endpoint 2 is 64 bytes, and

therefore reception of 512 bytes packets requires

ST7 software intervention every 64 bytes. This

means that after a CTR interrupt the hardware

puts the Endpoint 2 status bits for the current di-

rection (transmit or receive) in NAK status. The

ST7 software must then write the status bits to

VALID when it is ready to transmit or receive new

data.

On the contrary, in Upload or Download mode, the

physical address of Endpoint 2 is automatically in-

cremented every 64 bytes until a 512-byte buffer is

full.

Toggling between the two buffers is automatically

managed as soon as 512 bytes have been trans-

mitted to USB (Upload mode) or received from

USB (Download), if the next buffer is available:

Otherwise, the endpoint is set to invalid until a

buffer has been released by the DTC.

11.3.4.2 Switching back to Normal Mode

The USB interface is reset by hardware in Normal

mode on reception of a packet with a length below

the maximum packet size. In this case, the few

bytes are received into one of the two 512-byte

buffers and the ST7 must process by software the

data received. For this purpose, the information in-

dicating which 512-byte buffer was last addressed

is given to the ST7 by the USB Data Buffer Manag-

er (BUFNUM bit in the BUFCSR register), and the

number of received bytes is obtained by reading

the USB interface registers. With these two items

of information, the ST7 can determine what kind of

data has been received, and what action has to be

taken.

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